The invention relates to an optical system for reflective mode light valve full colour display, and more particularly to a compact and highly optically efficient trichroic prism assembly (TPA), and to a polarizing beam splitter (PBS). This TPA and PBS combination is placed between the objective lens and the LCD panel or other kind of reflective light valves to form a compact projection system.
Conventional projection displays employ mainly transmittive light valves, such as active matrix LCD panels. On the other hand, reflective made light valves offer many advantages such as high aperture ratio, high light efficiency and good projection image quality. Large scale projection systems based on reflective liquid crystal light valves (LCLV) have been successfully made and deployed. More recently, crystalline silicon based reflective mode CMOS liquid crystal light valves are available. They offer the additional advantages of full integration of CMOS circuits on the chip and economy of scale in mass production. Compact optical systems are needed for this new class of light valves.
In a reflective projector system, a collimated light source is first split into three primary colours by two dichroic colour filers (usually with a first blue filter and a second red filter). Then these light beams are directed onto the corresponding sight valves along different optical paths. The reflected light beams, having changed polarizations, are then recombined using two dichroic filters. These filters can be the same set of colour separating filters or different ones. The reflected light is separated from the incident light using a PBS and finally projected onto the screen. U.S. Pat. No. 4,687,301 disclosed one such colour separation-recombining optical assembly. Dichroic filters immersed in index matching fluid are used for both colour separation and recombination. The angle of incidence on the blue filter is 24xc2x0 while it is 12xc2x0 on the red filter.
U.S. Pat. No. 4,969,730 describes a 3-prism assembly which is commonly known as a colour splitting prism. This prism acts as both a colour separator and a colour recombiner. It is in principle the same as the invention disclosed in U.S. Pat. No. 4,687,301, but much improved in terms of ease of fabrication. The blue filter and red filter are coated onto the surfaces of prisms. The angles of incidence are all 30xc2x0. A PBS is also used to separate the incident beam from the polarization modulated reflected beam.
U.S. Pat. No. 5,644,432 describes a projection system where the colour separator and recombiner consist of the same 3-prism assembly. A PBS is used to separate the incident and reflected light beams. In this case, there is no air gap in the blue filter so that the 3 prisms can be glued together. The blue and red dichroic filters have large angles of incidence of 30xc2x0 in order to maintain a short back working distance for the projective lens.
The optical system of a full colour reflective made LCLV projection, must have the following characteristics: (1) Large output light flux, which means large system optical invariance, or system etandue with LCLV, (2) Dual-polarization utilization, (3) High efficiency colour separation and recombination which is polarization independent, and (4) Compact and small retrofocus for the projection lens. The colour separation and recombination prisms disclosed in the prior art do not meet these requirements. It is accordingly an object of the invention to provide a colour splitting recombining prism with small incident angles for dichroic coatings.
Current liquid crystal (LCD) projection displays are based mainly on transmittive LCD light valves as the image generator. The drawback of this kind of LCD projector is that the aperture ratio (AR) of the LCD panel is small. It gets smaller as the resolution of the light valve increases. For example, the AR is about 0.67 for SVGA displays, and is about 0.5 for XGA LCD panels. In addition to low light efficiency, low AR also requires a black matrix to hide the transistors, which produces pixelation. Depixelization is therefore necessary, adding complexity to the optical system design. In this type of transmittive projector different sets of color filters are used for the separation of the input light into three primary colors and for recombining them after going through the LCD light valves. The color recombiner is usually performed with the X-cube.
Reflective mode silicon CMOS liquid crystal light valves can overcome the drawback of low AR in transmittive LCD panels. The AR of silicon based CMOS LCD can be as high as 92%, regardless of the resolution. It is because that the transistor can be hidden beneath the reflective mirror on the pixel. Thus the light efficiency and the quality of projected image can be greatly improved.
The projector optics of the reflective light valve is decidedly more complicated than transmittive ones. One consideration is the change of s and p polarization in the (AR) light path.
A full color projector can either be a time sequential type employing one reflective LCD panel or a 3-pastel type with all 3 primary colors on at the same time. This application is concerned with the latter. Such projectors generally require an optical sub-system to separate the primary colors from the input white light source (typically an arc lamp), and another sub-system to recombine the 3 primary colors after modulation by the reflective light valves. The color separator and the color recombiner can be the same item of optics or they can be physically different. For a compact projection system, the latter is much preferred.
There are several designs for the optical subassembly for reflective light valve based color projectors. The basic element for reflective light valve is a polarizing beam splitter (PBS), which reflects s-polarized light and transmit p-polarized light. In the must straightforward design, 3 PBS can be used for the 3 primary color panels. Color separation and color recombining can be done in separate sets of filters similar to the transmittive projectors. FIG. 1 shows the basic setup. Dichroic reflectors are used to reflect red and blue lights. The R, G, B channels are seat to the PBS and the 3 light valves. The image-modulated reflected lights are then sent to the color recombining X-prism for projection. Because the reflective coatings in the X-prism work best far s-polarized light, half wave plates are usually needed for the red and blue channels to rotate the p-polarized light from the reflective LCD. This system is thus quite complex
According to a first aspect of the invention there is provided a polarizing beam splitter having s non-cubic configuration, the arrangement being such that angles of incidence on dichroic coatings or filters may be maintained of small value.
The splitter may be trapezoidal in configuration.
According to a second aspect of the invention there is provided apparatus for full colour projection display, including a prism assembly adapted to function as a colour separator and colour recombiner, and a plurality of reflective light valves.
The prism assembly may be trichroic.
The following calculations illustrate the invention, which can provide an optical sub-system where the angles of incidence of the light beams an the dichroic coatings are kept as small as possible. This is due to the phenomenon of polarization separation. A dielectric multilayer optical coating used to form the dichroic filter always consists of periodic stacks of high refractive index layer (H) and low reflective index layers (L). For nHdH=nLdL=xcex/4, the reflected light is given by the expression:                                           Δ            ⁢                          xe2x80x83                        ⁢                          λ              R                                λ                =                              4            π                    ⁢                      sin                          -              1                                ⁢                      "LeftBracketingBar"                                                            η                  H                                -                                  η                  L                                                                              η                  H                                +                                  η                  L                                                      "RightBracketingBar"                                              (        1        )            
where xcex94xcexR is the bandwidth of the reflected light xcex7X and xcex7L are the effective admittances of the H and L layers respectively. xcex7H and xcex7L are functions of the incident angle and the polarization state of the light. xcex7is=nicosxcex8i for s-polarization, and             η      ip        =                  η        i                    cos        ⁢                  xe2x80x83                ⁢                  θ          i                      ,
for p-polarization. Here i=H, or L and xcex8i is the refractive angle in each film. The relation between xcex8i and the incident angle xcex8 is given by the usual relationship npsinxcex8=nisinxcex8i.
A prism assembly and non-polarizing beam splitter embodying the invention are hereinafter described, by way of example, with reference to the accompanying drawings.